1. Field of the Invention
The present invention relates to a laser irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser beam emitted from the laser to an object to be irradiated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser beam irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light emitting device and an electronic device which includes the electro-optical device as a part.
2. Description of the Related Art
In recent years, a wide study has been made on a technique in which laser annealing is performed for a semiconductor film formed on an insulating substrate made of glass or the like, to crystallize the film, to improve its crystallinity so that a crystalline semiconductor film is obtained, or to activate an impurity element. Note that a crystalline semiconductor film in this specification indicates a semiconductor film in which a crystallized region is present, and also includes a semiconductor film which is crystallized as a whole.
A method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on a surface to be irradiated, and scanning the laser beam (or relatively shifting an irradiation position of the laser beam with respect to the surface to be irradiated) to conduct annealing is superior in mass productivity and is excellent in technology. The “linear shape” described here means not a “line” in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, it indicates a shape having an aspect ratio of 2 or more (preferably, 10 to 10000). Note that the linear shape is used to obtain an energy density required for sufficiently annealing an object to be irradiated.
FIG. 7 shows an example of a configuration of an optical system for forming laser beam in a linear shape on a surface to be irradiated. This configuration is extremely general. All optical systems described above are based on the configuration shown in FIG. 7. According to the configuration, a cross sectional shape of laser beam is converted into a linear shape, and simultaneously an energy density distribution of laser beam on the surface to be irradiated is homogenized. In general, an optical system for homogenizing the energy density distribution of laser beam is called a beam homogenizer.
Laser beam emitted from a laser 71 is divided in a direction perpendicular to a traveling direction thereof by a cylindrical lens group 73, thereby determining a length of linear laser beam in a perpendicular direction. The direction is called a first direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the first direction is changed in accordance with a direction of light bent by the mirror. In the configuration, the cylindrical lens array is divided into seven parts. Then, the laser beams are synthesized on a surface to be irradiated 79 by a cylindrical lens 74, thereby homogenizing an energy density distribution of the linear laser beam in the longitudinal direction.
Next, the configuration shown in the side view of FIG. 7 will be described. Laser beam emitted from a laser 71 is divided in a direction perpendicular to a traveling direction thereof and the first direction by cylindrical lens arrays 72a and 72b, thereby determining a length of linear laser beam in a width direction. The direction is called a second direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the second direction is changed in accordance with a direction of light bent by the mirror. In this configuration, the cylindrical lens arrays 72a and 72b each are divided into four parts. The divided laser beams are temporarily synthesized by a cylindrical lens 74. After that, the laser beams are reflected by a mirror 77 and then condensed by a doublet cylindrical lens 78 so that they become again single laser beam on the surface to be irradiated 79. The doublet cylindrical lens 78 is a lens composed of two cylindrical lenses. Thus, a homogenized energy density distribution of the linear laser beam in a width direction and a length of the linear laser beam in a width direction are determined.
For example, an excimer laser in which a size in a laser window is 10 mm×30 mm (which each are a half-width in beam profile) is used as the laser 71 and laser beam is produced by the optical system having the configuration shown in FIGS. 7A and 7B. Then, linear laser beam which has a uniform energy density distribution and a size of 125 mm×0.4 mm can be obtained on the surface to be irradiated 79.
At this time, when, for example, quartz is used for all base materials of the optical system, high transmittance is obtained. Note that coating is preferably conducted for the optical system such that transmittance of 99% or more is obtained at a frequency of the used excimer laser.
Then, the linear laser beam formed by the above configuration is irradiated with an overlap state while being gradually shifted in a width direction thereof. Thus, when laser annealing is performed for the entire surface of an amorphous semiconductor film, the amorphous semiconductor film can be crystallized, crystallinity can be improved to obtain a crystalline semiconductor film, or an impurity element can be activated.